Photos are saved on the Mac so no need for iCloud and not always access to internet
iCloud Photo Library is also a single user library and not meant for sharing a library.
Photos has been designed to share selected photos, but not the library. Each user maintains an own library and shares the best, selected photos with friend and family.
The work-around for sharing a library between users, that worked in iPhoto or Aperture does no longer work well in Photos, if you want to to share the system photo library (iPhoto: Sharing libraries among multiple users). To make that work in Photos you have to log off, before the other user can open the library.
I understand, that you do not want to use an external drive with a portable Mac. You could partition the system drive, if your Mac has a large internal drive. If the internal drive is small, you will sooner or later need an external drive for your photos anyway.